rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af, iruserok_af,
   ruserok_af - routines for returning a stream to a remote command


   #include <netdb.h>   /* Or <unistd.h> on some systems */

   int rcmd(char **ahost, unsigned short inport, const char *locuser,
            const char *remuser, const char *cmd, int *fd2p);

   int rresvport(int *port);

   int iruserok(uint32_t raddr, int superuser,
                const char *ruser, const char *luser);

   int ruserok(const char *rhost, int superuser,
               const char *ruser, const char *luser);

   int rcmd_af(char **ahost, unsigned short inport, const char *locuser,
               const char *remuser, const char *cmd, int *fd2p,
               sa_family_t af);

   int rresvport_af(int *port, sa_family_t af);

   int iruserok_af(const void *raddr, int superuser,
                   const char *ruser, const char *luser, sa_family_t af);

   int ruserok_af(const char *rhost, int superuser,
                  const char *ruser, const char *luser, sa_family_t af);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   rcmd(),    rcmd_af(),    rresvport(),    rresvport_af(),    iruserok(),
   iruserok_af(), ruserok(), ruserok_af():
       Since glibc 2.19:
       Glibc 2.19 and earlier:


   The  rcmd() function is used by the superuser to execute a command on a
   remote machine using an authentication scheme based on privileged  port
   numbers.   The  rresvport()  function  returns  a  file descriptor to a
   socket with an address in the privileged port  space.   The  iruserok()
   and  ruserok()  functions  are  used by servers to authenticate clients
   requesting service with rcmd().  All four functions  are  used  by  the
   rshd(8) server (among others).

   The  rcmd()  function  looks up the host *ahost using gethostbyname(3),
   returning -1 if the host does not exist.  Otherwise, *ahost is  set  to
   the  standard  name  of  the  host and a connection is established to a
   server residing at the well-known Internet port inport.

   If the connection succeeds, a socket in the  Internet  domain  of  type
   SOCK_STREAM  is returned to the caller, and given to the remote command
   as stdin and stdout.  If fd2p is nonzero, then an auxiliary channel  to
   a  control process will be set up, and a file descriptor for it will be
   placed in *fd2p.  The control process  will  return  diagnostic  output
   from  the  command (unit 2) on this channel, and will also accept bytes
   on this channel as being UNIX signal numbers, to be  forwarded  to  the
   process group of the command.  If fd2p is 0, then the stderr (unit 2 of
   the remote command) will  be  made  the  same  as  the  stdout  and  no
   provision  is made for sending arbitrary signals to the remote process,
   although you may be able to get  its  attention  by  using  out-of-band

   The protocol is described in detail in rshd(8).

   The  rresvport()  function is used to obtain a socket with a privileged
   port bound to it.  This socket  is  suitable  for  use  by  rcmd()  and
   several  other functions.  Privileged ports are those in the range 0 to
   1023.  Only a privileged process (on Linux:  a  process  that  has  the
   CAP_NET_BIND_SERVICE  capability  in  the  user namespace governing its
   network namespace).  is allowed to bind to a privileged port.   In  the
   glibc  implementation,  this function restricts its search to the ports
   from 512 to 1023.  The port argument  is  value-result:  the  value  it
   supplies  to  the  call  is  used  as the starting point for a circular
   search of the port range; on (successful) return, it contains the  port
   number that was bound to.

   iruserok() and ruserok()
   The  iruserok() and ruserok() functions take a remote host's IP address
   or name, respectively, two usernames and a flag indicating whether  the
   local  user's  name is that of the superuser.  Then, if the user is not
   the superuser, it checks the /etc/hosts.equiv file.  If that lookup  is
   not  done,  or  is  unsuccessful,  the .rhosts in the local user's home
   directory is checked to see if the request for service is allowed.

   If this file does not exist, is not a regular file, is owned by  anyone
   other  than the user or the superuser, is writable by anyone other than
   the owner, or is hardlinked anywhere, the  check  automatically  fails.
   Zero is returned if the machine name is listed in the hosts.equiv file,
   or the host  and  remote  username  are  found  in  the  .rhosts  file;
   otherwise  iruserok() and ruserok() return -1.  If the local domain (as
   obtained from gethostname(2)) is the same as the  remote  domain,  only
   the machine name need be specified.

   If  the  IP  address  of the remote host is known, iruserok() should be
   used in preference to ruserok(), as it does not  require  trusting  the
   DNS server for the remote host's domain.

   *_af() variants
   All  of the functions described above work with IPv4 (AF_INET) sockets.
   The "_af" variants take  an  extra  argument  that  allows  the  socket
   address  family  to be specified.  For these functions, the af argument
   can be specified  as  AF_INET  or  AF_INET6.   In  addition,  rcmd_af()
   supports the use of AF_UNSPEC.


   The  rcmd()  function returns a valid socket descriptor on success.  It
   returns -1 on error and prints a diagnostic  message  on  the  standard

   The  rresvport()  function  returns a valid, bound socket descriptor on
   success.  It returns -1 on  error  with  the  global  value  errno  set
   according  to  the  reason  for  failure.   The  error  code  EAGAIN is
   overloaded to mean "All network ports in use."

   For information on the return from ruserok() and iruserok(), see above.


   The   functions   iruserok_af(),   rcmd_af(),    rresvport_af(),    and
   ruserok_af() functions are provide in glibc since version 2.2.


   For   an   explanation   of   the  terms  used  in  this  section,  see

   │InterfaceAttributeValue          │
   │rcmd(), rcmd_af()           │ Thread safety │ MT-Unsafe      │
   │rresvport(), rresvport_af() │ Thread safety │ MT-Safe        │
   │iruserok(), ruserok(),      │ Thread safety │ MT-Safe locale │
   │iruserok_af(), ruserok_af() │               │                │


   Not in POSIX.1.  Present on the BSDs, Solaris, and many other  systems.
   These functions appeared in 4.2BSD.  The "_af" variants are more recent
   additions, and are not present on as wide a range of systems.


   iruserok() and iruserok_af() are declared in glibc headers  only  since
   version 2.12.


   rlogin(1), rsh(1), intro(2), rexec(3), rexecd(8), rlogind(8), rshd(8)


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at


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